It is required that coating agents be capable of imparting design freedom to the surface of various base materials as well as forming a film having a property of preventing deterioration of the base material surface. Particularly, in recent years, a coating agent capable of forming a film having water resistance or solvent resistance and excellent chemical resistance to a level of possibly preventing deterioration of the base material due to attachment of chemicals in, for example, cleaner or acidic rain, is demanded from the industrial field.
A coating agent having the above-described properties is frequently demanded for the purpose of surface protection of plastic products which are prone to deteriorate particularly due to the attachment of organic solvents or chemicals.
However, while there are a wide variety of materials constituting the plastic products such as acrylonitrile-butadiene-styrene resins (ABS resins), polycarbonate resins (PC resins), polystyrene resins (PS resins) and polymethyl methacrylate resins (PMMA resins), a coating agent having excellent adhesiveness even to base materials which are generally called sparingly adhesive base materials, such as ABS resins, and being capable of forming a film having excellent water resistance, solvent resistance or chemical resistance, is not yet known.
Since many of the plastic base materials generally undergo plasticization by heat, in order to form a coating film by applying various coating agents on the surface of the plastic base material, the drying temperature generally needs to be set at 60° C. or below.
However, under such low temperature conditions as described above, usually, the crosslinked structure between the binder resins constituting the coating film is not formed sufficiently, and as a result, a coating film having a high level of solvent resistance or water resistance may not be formed in some cases.
On the other hand, concomitantly with the recent increase in the demand for metals, the demand for a coating agent for surface protection of metallic base materials including steel sheets is also increasing. Such a coating agent is required to have, in addition to the solvent resistance as mentioned above, corrosion resistance to a level capable of preventing peeling or swelling of coating film due to the corrosion of metallic base materials, or a high level of chemical resistance. In particular, in the steel industry where cleaning of the coating film surface formed on the surface of a metallic base material using an alkaline cleaner or the like is frequently implemented, chemical resistance is an important property for preventing peeling or dissolution of coating film under the effect of the cleaner, deterioration of the metallic base material and the like.
As a method of forming a coating film with excellent chemical resistance, there is known a method of rapidly forming a crosslinked structure between binder resins by performing the drying process at a high temperature of about 200 to 250° C.
However, in the steel industry in recent years, there is a tendency of setting the drying temperature generally at a low temperature of about 80° C. and setting the drying time very short in the process of forming a coating film by applying a coating agent on the surface of a metallic base material and drying, in order to enhance the production efficiency for the metallic material. Under such conditions, it is difficult to obtain a coating film having excellent chemical resistance.
Coating agents capable of forming a coating film having the above-mentioned various properties have been examined for long, and for example, there is known an aqueous dispersion of a curable polymer containing polymer particles, a polyfunctional epoxy compound and a water-based medium, wherein the polymer particles are composed of at least two kinds of layers, in which phase A is composed of a carboxyl group-containing polymer, and phase B is an ethylenic polymer having a basic nitrogen atom-containing group (see, for example, Patent Citation 1).
However, in the aqueous dispersion of a curable polymer, it was often difficult to form a coating film having good adhesiveness to various plastic base materials such as those described above. Furthermore, the water resistance and solvent resistance of the coating film formed using the dispersion also do not satisfy the high level required in practice, and particularly, it is often difficult to form a coating film having excellent chemical resistance or corrosion resistance that are required of surface coating agent for metallic base materials.
As another coating agent, there is known an aqueous coating agent containing a vinyl-urethane-based copolymer in which the chain part of a vinyl-based polymer and the chain part of a urethane-based polymer are linked via a linking unit having bonds between silicon atoms and oxygen atoms (see, for example, Patent Citation 2).
However, since this aqueous coating agent has a relatively high crosslinking density inside the particles formed from the vinyl-urethane-based copolymer, it is difficult for fusion between the particles to occur during film formation, and as a result, it is not feasible in some cases to form a coating film having excellent solvent resistance, which is formed uniformly and densely.
Moreover, the aqueous coating agent is still unsatisfactory in terms of the adhesiveness to the plastic base materials, and in the coating film formed by using the aqueous coating agent, peeling or dissolution occurs due to the attachment of water or chemicals. Consequently, corrosion or deterioration of the base material may not be sufficiently prevented.    [Patent Citation 1] Specification of Japanese Patent Application Laid-Open Publication No. 9-235332    [Patent Citation 2] Pamphlet of International PCT Application No. 2005/054341